Cut your potato into slices however large you would like the width of the cubes to be. Keep all of your cuts the same width to achieve uniform cubes.
Place your slices cut side down and slice again lengthwise into the same width.
Holding your slices in tact with your free hand, rotate the potato a quarter turn and make the final cuts again in the same width in the opposite direction of your previous cuts. You will be left with uniform pieces of cubed potatoes.
If you feel comfortable, you can stack multiple potato slices to quicken the process.